1. Field of the Invention
The invention relates to batteries, and particularly to batteries having two modes of operation.
2. Description of Prior Art
A battery is classified as either a primary battery or a secondary battery. A primary battery is one in which the stored energy is released in an irreversible process, and the battery is depleted when the total amount of stored energy is released. A secondary battery is one in which the stored energy is released in a reversible process, and the battery is capable of being repeatedly charged and discharged.
A reserve battery is a primary battery that can be stored for long periods of time prior to discharge and that requires some form of activation to bring it to a full operational state. The reserve battery is inhibited from open circuit self-discharge during the pre-activation state by having the electrolyte stored separately from the electrodes or by having the electrolyte infused into the plate stack of the battery in a non-conductive state.
Aerospace and defense applications usually employ a thermal battery as the reserve battery. For the thermal battery, the electrolyte permeates the plate stack as a solid state salt and is non-conductive for the range of storage temperatures in the pre-activation state. The thermal battery is activated by the ignition of an internal pyrotechnic that heats the electrolytic salt to a liquid state. In the liquid state, the electrolyte conducts current, and the thermal battery is activated.
In other reserve battery designs, the plate stack is dry, and the electrolyte is stored in liquid form in a separate storage reservoir. Upon activation, the electrolyte is injected into the plate stack allowing the battery to discharge current into a load attached to the terminals of the battery.
Lithium thionyl chloride (Li/SOCl.sub.2) and other lithium-type batteries are widely used in commercial applications as primary batteries. As primary batteries, lithium batteries have substantial storage capacity and are capable of long-duration storage. However, lithium batteries have a limited rate of discharge because the non-corrosive electrolyte is not highly conductive. Under open circuit conditions, a passivation layer accumulates on the surface of the electrodes in the plate stack. The passivation layer protects against deterioration of the electrodes but prevents a high rate discharge for the lithium battery.
Lithium batteries are commercially used in watches, computers, and other electronic applications as low-drain, high capacity storage cells and are designed for long storage and operation life. However, because of their limited discharge rates, lithium batteries are not used with apparatuses that require short duration and high current drain, such as those apparatuses that normally employ thermal batteries.
Because of the high storage capacity of a lithium battery, a lithium battery can be used as a reserve battery if its discharge rate can be substantially increased. It is known that the discharge rate of a lithium battery can be substantially increased by increasing the acidity of the electrolyte through a substantial increase in the molar concentration of the acidic salts in the electrolyte. This acidic form of the electrolyte can support a much higher discharge rate for the battery, but is also highly corrosive to the electrodes, which limits battery life. Examples of using a lithium battery as a reserve battery include: D. L. Miller et al., "Long Life Reserve Li/SOCl.sub.2 Battery," IEEE, 0-7803-2459-5/95, 1995, pp. 49-51; N. A. Remer et al., "Development and Manufacture of a Large, Multi-Cell Lithium Thionyl Chloride Reserve Battery," IEEE, 0-7803-0552-3/92, 1992, pp. 77-80; J. C. Hall, "Performance and Safety to NAVSEA Instruction 9310.1A of Lithium-Thionyl Chloride Reserve Batteries," The 1993 Goddard Space Flight Center Battery Workshop, Nov. 15-17, 1983, pp. 149-158; C. T. Dils et al., "Reserve Li/SCl.sub.2 Battery Safety Testing," The 1993 Goddard Space Flight Center Battery Workshop, Nov. 15-17, 1983, pp. 139-144.
In the prior art, lithium batteries have been configured for use as primary reserve batteries by storing the highly acidic form of the electrolyte in a separate reservoir and then injecting the electrolyte into the dry electrode plate stack to activate the battery. For example, with a cylindrical reserve lithium battery, the electrolyte is stored in a separate reservoir adjacent to the plate stack and exterior to the battery. The separate reservoir is a cylindrical flexible container with bellowed walls. Prior to activation, the lithium reserve battery provides no power. Upon activation, a pyrotechnic gas generator is ignited, which drives a piston to collapse the container holding the electrolyte. The collapsing container forces the electrolyte into the plate stack.
In an example from the prior art, the bellowed walls are in a collapsed state in an electrolyte container prior to activation. The bellowed walls expand under gas pressure during activation to force the electrolyte into an adjacent plate stack. Prior to activation the battery provides no power. N. A. Remer et al., "Development and Manufacture of a Large, Multi-Cell Lithium Thionyl Chloride Reserve Battery," IEEE, 0-7803-0552-3/92, 1992, pp. 49-51.
In another example from the prior art, electrolyte is stored in a central cylindrical reservoir. The battery activation is initiated by an electrical pulse applied to an igniter which fires a gas generator. The expanding gas moves a piston which pressurizes the electrolyte. The increased pressure subsequently breaks a containment burst diaphragm and forces the electrolyte into a plate stack. The plate stack consists of series connected horseshoe shaped cells surrounding the central cylindrical reservoir of electrolyte. Prior to activation the battery provides no power. D. L. Miller et al., "Long Life Reserve Li/SOCl.sub.2 Battery," IEEE, 0-7803-2459-5/95, 1995, pp. 75-80.
In another example from the prior art, a pressure plenum containing Freon.RTM. gas surrounds the outside of a bellows assembly and provides the pressure to collapse the bellows assembly. The collapsing bellows assembly transfers electrolyte to the plate stack. The battery activation is initiated by an electrical pulse applied to a cutter. The cutter bursts a nickel diaphragm, and the Freon.RTM. gas is allowed to collapse the bellows assembly. Prior to activation the battery provides no power. C. T. Dils et al., "Reserve Li/SOCl.sub.2 Battery Safety Testing," The 1983 Goddard Space Flight Center Battery Workshop, Nov. 15-17, 1983, pp. 139-144.
In another example of the prior art, a battery is used in an undersea application. As the battery sinks, the pressure of the sea water is used to burst a disk above a piston forcing electrolyte into a dry plate stack using the hydrostatic pressure. Prior to activation the battery provides no power. J. C. Hall, "Performance and Safety to NAVSEA Instruction 9310.1A of Lithium-Thionyl Chloride Reserve Batteries," The 1983 Goddard Space Flight Center Battery Workshop, Nov. 15-17, 1983, pp. 149-158.
These prior art lithium reserve batteries suffer from at least three drawbacks. First, the prior art batteries require much space because a separate reservoir is required to contain the separated electrolyte. Second, a large void space is created in the battery enclosure when the electrolyte is pumped into the dry plate stack by the pyrotechnic gas generator, the expanding Freon.RTM. gas, or the sea water used to displace the piston driving the electrolyte into the dry plate stack. Third, the prior art batteries have a single high power mode of operation and are incapable of sustaining power prior to activation because the plate stack is kept dry. The prior art batteries supply no power prior to being activated.